The present invention relates to a telephone line transmitter for transmitting data over telephone lines. More particularly, the present invention relates to a low-powered remote sensor and telephone line transmitter which does not require any external power supply, other than being interconnected to a telephone line.
Telephone line data transmitting systems require some type of external power supply. Typically, this will either be a 120 volt AC power supply or a DC battery. This necessitates either the placement of a distribution transformer at each of the transmitting units (in the case of the 120 volt AC power supply) or periodic servicing of a battery at the transmitting unit (in the case of a DC battery). Other power supply sources, such as solar cells, also require periodic maintenance and are subject to vandalism or the like.
Additionally, the requirements for an FCC certification, in particular, a Part 68 registration, are very stringent. They require that the transmitter unit exhibit more than 10 megaohms at 100 volts DC when "on hook." In addition, there are stringent requirements for lightening and surge protection. When a ring signal is received, the transmitter unit must exhibit 40K ohms to 1600 ohms on hook AC impedance for the standard ring frequency of 16 to 68 hertz and 40 to 150 volts. When the transmitter unit goes "off hook", it must provide a resistance less than 200 ohms at 600 ohms reflected impedance. As a practical matter, this means that the unit must operate down to 2 volt DC line voltages.
Another problem often encountered in developing a remote telephone line transmitter unit is the temperature instability frequently exhibited by voltage to frequency converters.
The present invention overcomes these and many other problems associated with remote telephone line transmitter systems.